This paper discusses some conditions necessary to detect an effect of maternal nutrition on birth weight and the relative contribution of calories and protein to such an effect. The expected dose- and time-response relationships for nutritional interventions aimed at the improvement of birth weight are also discussed. There appears to be a minimal level of nutrients which must be available in order to obtain adequate birth weight. However, above this minimum level, pregnant women can adapt themselves to a wide variety of food intake, both in quantity and quality, without affecting birth weight. The relative contribution of calories and protein to an increase in birth weight depends on the limiting nutrients of the home diet in the population under study. Other factors like physical activity, prevalence of disease and magnitude of the maternal nutritional stores before pregnancy are also important determinants of the relative contribution of calories and protein to birth weight. The anticipated input of a nutritional intervention on birth weight should range between 25 and 84 g of birth weight/10,000 kcal ingested during pregnancy. This estimate was computed from analyses based on four sources of published data: weight gain during pregnancy, prepregnant weight, fetomaternal body composition, and food intake during pregnancy. The expected reduction in proportion of low birth weight (LBW less than 2.5 kg) babies following a nutritional intervention will depend not only on the estimated range of fetal weight increase but also on the total amount of supplemented calories ingested during pregnancy as well as on the existent proportion of low birth weight babies prior to the intervention. The offspring of women who have low prepregnant weight, poor diet, low level of replacement of the home diet by the supplement, low physical activity during pregnancy and good health status will show larger increase in birth weight per unit of supplemented calories. Finally, nutritional interventions during pregnancy as opposed to earlier in the life of the mother, should have the higher impact on birth weight. In consequence, interventions as of pregnancy are recommended.This paper discusses some conditions necessary to detect an effect of maternal nutrition on birthweight and the relative contribution of calories and protein to such an effect. The expected dose- and time-response relationships for nutritional interventions aimed at the improvement of birthweight are also discussed. There appears to be a minimal level of nutrients which must be available in order to obtain adequate birthweight. However, above this minimum level, pregnant women can adapt themselves to a wide variety of food intake, both in quantity and quality, without affecting birthweight. The relative contribution of calories and protein to an increase in birthweight depends on limiting nutrients of the home diet in the population under study. Other factors like physical activity, prevalence of disease, and magnitude of the maternal nutritional stores prior to pregnancy are also important determinants of the relative contribution of calories and protein to birthweight. The anticipated input of a nutritional intervention on birthweight should range between 25-84 g of birthweight/10,000 kcal ingested during pregnancy. This estimate was computed from analyses based on 4 sources of published data: weight gain during pregnancy, prepregnant weight, fetomaternal body composition, and food intake during pregnancy. The expected reduction in proportion of low birthweight (LBW or= 2.5 kg) babies following a nutritional intervention will depend not only on the estimated range of fetal weight increase but also on the total amount of supplemented calories ingested during pregnancy as well as on the existent proportion of low birthweight babies prior to the intervention. The offspring of women who have low prepregnant weight, poor diet, low level of replacement of home diet by the supplement, low physical activity during pregnancy, and good health status will show larger increase in birthweight/unit of supplemented calories. Finally, nutritional interventions during pregnancy as opposed to earlier in the life of the mother, should have the highest impact on birthweight. Consequently, interventions as of pregnancy are recommended.